Method and apparatus for simplifying the control of a switch

ABSTRACT

A circuit for use in a half bridge converter includes a high side switch coupled between a positive input terminal and a first terminal of a primary transformer winding. A low side switch is coupled between a negative input terminal and the first terminal. A first control circuit is coupled to the high side switch to sense a slope of a voltage across the high side switch while the high side switch is off to control the high side switch in response to the sensed slope across the high side switch. A second control circuit is coupled to the low side switch to sense a slope of a voltage across the low side switch while the low side switch is off to control the low side switch in response to the sensed slope of the voltage across the low side switch.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/268,287, filed Nov. 10, 2008, now pending, which is a continuation ofU.S. patent application Ser. No. 11/474,163, filed Jun. 23, 2006, nowissued as U.S. Pat. No. 7,466,170, which is a divisional of U.S. patentapplication Ser. No. 10/675,824, filed Sep. 30, 2003, now issued as U.S.Pat. No. 7,091,752. U.S. patent application Ser. No. 12/268,287 and U.S.Pat. Nos. 7,091,752 and 7,466,170 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to switches and, morespecifically, the present invention relates to semiconductor switchesthat are coupled to control circuits.

2. Background Information

Electronic circuits using switches coupled to control circuits typicallyhave to be designed to meet cost targets defined at the start of adesign. The control circuitry used to control the switching of theswitches can be a significant part of the electronic circuit cost andany way to reduce the cost of this circuitry is therefore a benefit.

One circuit configuration using switches coupled to control circuits isa power conversion circuit called a half bridge converter. This circuittypically has two switches configured in a half bridge wherein oneswitch is referred to as a high side switch and the other is referred toas a low side switch as will be known to one skilled in the art.

The control of power delivery to the load in this type of circuit istypically achieved by a control circuit controlling the period of timefor which each switch is on within a switching cycle. This on timeperiod is typically controlled in response to a feedback signal receivedfrom a sense circuit coupled to the load. The frequency of theswitching, being the reciprocal of the time between the start of one ontime period and the start of the next on time period of one of theswitches, is determined either by an oscillator forming part of thecontrol circuit or a resonant frequency of the load circuitry coupled tothe low side and high side switches.

In circuits where the oscillator forms part of the control circuit, turnon and or turn off signals are typically coupled to the high side switchin the half bridge by level shift circuitry, as will be known to oneskilled in the art, which adds significant cost to the overall halfbridge circuit. In circuits where the frequency of switching isdetermined by a resonant frequency of the load circuitry coupled to thelow side and high side switches, the turn on and or turn off signals canagain be coupled to the high side switch by level shift circuitry orusing inductively coupled drive signals, which typically sense thecurrent or voltage at the load to determine the time at which to turn onthe low side and high side switches.

Since the inductively coupled drive signals can be coupled with separatewindings and therefore individually referenced, to the low side and highside switch control circuits, the requirement for other level shiftcircuitry is eliminated. However the inductively coupled element isrequired which is coupled to the load and the control circuitry for bothlow side and high side switches, which again adds cost to the overallhalf bridge circuit.

In other configurations, inductively coupled drive signals can be usedto determine the start of an on time period of either the high side orlow side switches and individual oscillators coupled to both low sideand high side switches are used to determine the time at which the lowside or high side switches are turned off. Again, in theseconfigurations, an inductively coupled drive circuit and an oscillatoror timing circuit is required for both high side and low side switches,which adds cost. The same limitations apply to full bridge circuits andother configurations requiring a high side switch.

SUMMARY OF THE INVENTION

A method and apparatus for simplifying the control of a switch isdisclosed. In one embodiment, a circuit according to the teachings ofthe present invention includes a first switch coupled between an inputand a load. The circuit also includes a control circuit coupled to thefirst switch. The control circuit is adapted to control the first switchas a function of a voltage across the first switch. In anotherembodiment, a half bridge circuit according the teachings of the presentinvention includes a low side switch, a high side switch coupled to thelow side switch, a low side capacitor coupled to the low side switch, ahigh side capacitor coupled to the low side capacitor and the high sideswitch and a load connected between a junction between the low sideswitch and high side switch and a junction between the low sidecapacitor and the high side capacitor. The high side switch is adaptedto be turned on for a high side on time period when a voltage across thehigh side switch crosses below a threshold in response the low sideswitch turning off. The low side switch is adapted to be turned on for alow side on time period following a delay time after a voltage acrossthe low side switch crosses below the threshold in response to the highside switch turning off. In yet another embodiment, a circuit accordingto the teachings of the present invention includes a switching circuitcoupled to a load for applying a voltage of a first polarity across theload during a first on time period and applying a voltage of theopposite polarity during a second on time period. The circuit alsoincludes a sensing circuit coupled to load for sensing the voltageacross the load for a first sense period during the first on period andfor a second sense period during the second on period. The switchingcircuit is controlled to maintain a magnitude of the voltage across theload during the first on period substantially equal to the voltageacross the load during the second on period. Additional features andbenefits of the present invention will become apparent from the detaileddescription, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic illustrating a half bridge circuit where the drivesignals for the high side switch are coupled to the high side switch bylevel shift circuitry.

FIG. 2 is a schematic illustrating a half bridge circuit where the drivesignals for the high side and low side switches are coupled to the highside and low side switches by an inductively coupled drive circuit

FIG. 3 is one embodiment of a simplified control circuit to control aswitch in accordance with the teachings of the present invention.

FIG. 4 is another embodiment of a simplified control circuit to controla switch in accordance with the teachings of the present invention.

FIG. 5 is yet another embodiment of a simplified control circuit tocontrol a switch in accordance with the teachings of the presentinvention.

FIG. 6 is still another embodiment of a simplified control circuit tocontrol a switch in accordance with the teachings of the presentinvention.

FIG. 7 is a schematic diagram of one embodiment of a voltage sensecircuit that could be employed by a simplified control circuit tocontrol a switch in accordance with the teachings of the presentinvention.

DETAILED DESCRIPTION

A novel technique to implement a simplified control circuit to control aswitch is disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. In other instances, well-knownmaterials or methods have not been described in detail in order to avoidobscuring the present invention.

In general, a simple and novel technique for controlling the switchingof a switch is provided according to embodiments of the presentinvention. In one embodiment the switch is turned on for an on timeperiod in response to a voltage across the switch while the switch isoff or in response to the slope of the voltage across the switch whilethe switch is off. In one embodiment, the on time period of the switchis substantially fixed. In an embodiment where the switch is a high sideswitch, the switch is turned on in response to a voltage across theswitch while the switch is off or in response to the slope of thevoltage across the switch while the switch is off. In controlling theswitching of a switch in this way, complex circuitry normally requiredfor this control is eliminated reducing the cost of the circuitryrequired to perform this function.

To illustrate, FIG. 1 shows a schematic of a power conversion circuit100 using a half bridge configuration. Control circuit 113 is coupled toboth high side switch 102 and low side switch 103 which are both metaloxide semiconductor field effect transistors (MOSFETs) in the circuit ofFIG. 1. Switch 102 is referred to as a high side switch since it iscoupled to the high side or positive input rail 101 and the load 109. Itis coupled to low side return or ground rail 125 through the low sideswitch 103 or the load 109 and low side half bridge capacitor 126. Toturn MOSFET 103 on and off, the gate 117 of MOSFET 103 is driven withrespect to its source terminal 118, which is coupled to ground 114through current sense resistor 104. However, the gate 116 of MOSFET 102is driven with respect to the half bridge output terminal 115, which issubstantially at the Vin voltage 119 with respect to ground 114 whenswitch 102 is on and switch 103 is off. The driver and level shiftcircuit 105 therefore contains driver circuitry to drive the gates 116and 117 of switches 102 and 103 respectively and also level shiftcircuitry to generate a drive signal relative to the half bridge outputterminal 115, is applied to the gate 116 of MOSFET 102. The operatingfrequency, which is the reciprocal of the time between the start of oneon time period to the start of the next on time period of either switch102 or 103, is fixed by oscillator 121. The frequency of oscillator 121is set by resistor 122. The on time periods of switches 102 and 103 arenot fixed and are determined based on feedback voltage signal 106 andcurrent sense signal 120 which are coupled to control pulse widthmodulator (PWM) circuit 123. By varying the on time, the power deliveredto the load 109 is regulated.

Within the load circuit 109 is the transformer 108, which includesprimary or input winding 107 and secondary or output windings 110 and111. Circuit 112 at the output terminal Vout of power conversion circuit100 filters the output, which is necessary since with the PWM controlscheme of control circuit 113, the on time periods, and therefore alsothe off time periods, of switches 102 and 103 will vary depending on theoutput load represented by resistor 124. The varying off time periods ofswitches 102 and 103 would result in high output ripple voltages iffilter circuit 112 was not present. Other aspects of the convertercircuit 100 operation, necessary for the correct operation but notcritical to the disclosure of the present invention are not discussed.

FIG. 2 shows another schematic of a power conversion circuit 200 using ahalf bridge configuration. The load 205 is in this example is a lampwith a series coupled inductance Lp. In this example, the high side andlow side control circuits 201 and 202 and switches 203 and 204 areintegrated in separate high side and low side integrated circuits 206and 207 respectively. The configuration shown eliminates the need forlevel shift circuitry by using drive signals generated by windings L1and L2 that are magnetically coupled to Lp, to control the turn on ofswitches 203 and 204 respectively.

The integrated circuits 206 and 207 use external capacitors C5 and C6 todetermine the length of the on time period of the switches. Taking theintegrated circuit 206 as an example, during the on time period of highside switch 203, the capacitor C5 is charged with a fixed current. Whenthe voltage across C5 has reached a threshold level, the switch 203 isturned off.

As will be familiar to one skilled in the art, when switch 203 isswitched off, the voltage at the non dot end 210 of inductor Lpdecreases relative to the voltage at the dot end 211 of inductor Lp andthe voltage across switch 204 reduces as the voltage at the output ofthe half bridge 210 decreases relative to ground rail 214. Since windingL2 is magnetically coupled to inductor Lp, the voltage at the dot end213 of L2 becomes positive with respect to the voltage at the non dotend 212 of L2. This voltage across L2 is sensed by the low side controlcircuit 202 and the low side switch 204 is turned on. The on time periodof the low side switch is in turn determined by the value of C6 which ischarged with a constant current until the voltage across C6 reaches athreshold level at which point the low side switch 204 is turned off.

As will be known to one skilled in the art having the benefit of thisdisclosure, the voltage across inductor Lp then changes polarity and thewinding L1 coupled to the high side integrated circuit 206 provides aturn on drive signal to the high side control circuit 201, in the waydescribed above with L2 and the low side control circuit 202, and thesequence repeats. In the circuit shown in FIG. 2, the windings L1 and L2also provide the supply current to the integrated circuits 206 and 207,respectively. Other aspects of the converter circuit 200 operation,necessary for the correct operation but not critical to the disclosureof the present invention are not discussed.

Other possible circuit configurations include a variation on the circuitof FIG. 2 where the operation is truly resonant wherein the high sideand low side switches are turned off only when the voltage polarityacross the inductively coupled drive signal windings L1 and L2 changesdue to the resonant frequency of the LC resonant circuit of inductor Lpand capacitors C2 and C3. However, the need to generate drive signalsusing inductively coupled windings such as L1 and L2 remains unalteredand this configuration is therefore not discussed in detail so as not toobscure the teachings of the present invention.

FIG. 3 shows one embodiment of a circuit benefiting from the teachingsof the present invention. The circuit shown is a half bridge powerconversion circuit 300. The high side and low side control circuitry andswitches are shown as high side and low side integrated circuits 304 and305, respectively. The switches 315 and 324 are shown as MOSFETs in thecircuit of FIG. 3 though other types of switch can be used whilst stillbenefiting from the teachings of the present invention.

Since the internal circuitry is identical in high side and low sideintegrated circuits 304 and 305, the description below focuses on thehigh side integrated circuit 305. For the purposes of the descriptionbelow, the voltage across low side capacitor 322 is assumed to be equalto the voltage across high side capacitor 323, which is assumed to besubstantially fixed at a value of Vin/2 where Vin 327 is the voltageacross input terminals 328 and 301. Under this condition, and assumingthat the voltage across the high side and low side switches 315 and 324is substantially zero when they are on, the magnitude of the maximumvoltage applied across the transformer primary winding 318 is Vin/2 wheneither high side switch 315 or low side switch 324 are in the on state.

In the illustrated embodiment, the transformer primary winding 318 ismagnetically coupled with the output windings 325 and 326. The switch315 is coupled between the input 301 and the load which, in the circuitof FIG. 3, is the transformer 319 and all circuitry coupled between thetransformer output windings 325 and 326 and output terminals 330 and 331and any circuitry coupled between output terminals 330 and 331. In otherembodiments, not discussed here, the load could be a lamp circuitsimilar to the load 205 of FIG. 2, a motor, a motor winding or otherinductive load for example.

The switch 315 has three terminals, a first switch terminal 308, asecond switch terminal 307 and a control terminal 337. Since in thisdescription, switch 315 is a high side switch, terminals 308, 307 and337 could also be referred to as high side switch terminals. The controlcircuit 321, coupled to the switch 315 control terminal 337, includesswitch drive circuit 313, one shot circuit 309 and voltage sense circuit311. The operation of the circuit is described below.

For the purposes of this description, it is helpful to begin at aninitial condition where switch 315 is off, 324 is on and current isflowing through winding 318 from node 332 to node 314. At the end of theon time period of switch 324, the duration of which is determined by oneshot circuit 333, the switch 324 is turned off. Prior to switch 324turning off, the voltage across switch 315 is substantially equal to Vinaccording to the description above.

When the switch 324 turns off, the combination of leakage andmagnetizing energy stored in the leakage and magnetizing inductance ofwinding 318, causes the voltage at node 314 to rise with respect toterminal 328, as will be known to one skilled in the art having thebenefit of this disclosure. As the voltage at node 314 rises relative toterminal 328, the voltage across switch 315 drops. This voltage issensed by voltage sense circuit 311. When the voltage sense circuit 311senses the voltage across the switch 315 (across terminals 307 and 308)falls below a threshold value, the switch 315 is turned on by thecontrol circuit 321 for an on time period. In one embodiment the on timeperiod is substantially fixed. This threshold value can be definedduring the manufacture of the integrated circuit 304 or made to beadjustable by the user of integrated circuit 304 with circuitry notshown in FIG. 3.

In one embodiment, the switch 315 is not necessarily turned onimmediately when the voltage across switch 315 falls below the thresholdvalue. For instance, a delay time can be set within control circuitry321, for example within the voltage sense circuit 311, to delay theswitch 315 from turning on for a delay time. This delay time provides aminimum time between the time at which switch 324 turns off and the timewhen switch 315 turns on. This type of delay is typically used in halfbridge configurations of the type shown in FIG. 3 to avoid risk of highcurrents flowing through switches 315 and 324 simultaneously. This delaytime is often called a deadtime, which will be familiar to one skilledin the art and is therefore not detailed as a separate circuit block inthe circuit of FIG. 3.

In one embodiment, the on time period of switch 315 is fixed by the oneshot circuit 309. The duration of the on time period can be fixed duringthe manufacture of integrated circuit 304 or made to be adjustable bythe user of integrated circuit 304 with circuitry not shown in FIG. 3.At the end of the on time period of switch 315, the current intransformer winding 318 is flowing from node 314 to node 332.

When the switch 315 is on, the voltage across switch 324 issubstantially equal to Vin according to the description above. When theswitch 315 turns off, the voltage at node 314 falls with respect toterminal 328. This voltage is sensed by voltage sense circuit 334. Asthe voltage at node 314 falls relative to terminal 328, the voltageacross switch 324 drops. When the voltage sense circuit 311 senses thevoltage across the switch 324 falls below a threshold value, the switch324 is turned on by the control circuit 335 for an on time period. Inone embodiment the on time period of switch 324 is substantially fixed.This threshold value again can be defined during the manufacture of theintegrated circuit 305 or made to be adjustable by the user ofintegrated circuit 304 with circuitry not shown in FIG. 3. The operationcontinues according to the above description. As explained for the highside control circuit 321, the turning on of the switch 324 can bedelayed by a delay circuit within the voltage sense circuit 334 toprovide dead time between the turn off of switch 315 and turn on ofswitch 324.

In one embodiment, voltage sense circuit 311 can be designed to continuesensing the voltage across switch 315 while it is in the on state. Inthis way, fault conditions resulting in very high current flowingthrough the on resistance of the switch 315 can be detected since underthese conditions, the voltage across the switch 315 rises. A secondthreshold voltage in voltage sense circuit 311 can therefore be set tosense a fault condition of this type and turn the switch 315 from an onstate to an off state when the voltage drop across the switch crosses asecond threshold, before the end of the on time period defined by oneshot circuit 309. Since this is a fault condition, it is also oftennecessary to define a minimum off time period before the switch isallowed to turn on again which can be set within the control circuitry321. In one embodiment, operation of low side voltage sense circuit 334with respect to controlling low side switch 324 can be identical.

In the illustrated embodiment, the voltage sense circuits 311 and 334eliminate the requirement for inductively coupled windings such as L1and L2 in FIG. 2. Furthermore, the level shift circuitry containedwithin circuit block 105 in FIG. 1 is also not necessary. In theembodiment shown, regulator circuit 312, which is part of high sidecontrol circuit 321, charges capacitor 302 while switch 315 is in theoff state. The energy stored in capacitor 302 is used to provide theenergy required by integrated circuit 304 to operate. The regulatorcircuit 336 forming part of low side control circuit 335 performs asimilar function for low side integrated circuit 305. Control circuitsbenefiting from the teachings of the present invention do not howeverrequire internal regulator circuits such as 312 and 336 and the energyrequired for high side and low side switch operation, stored oncapacitors 302 and 303 can be sourced from an external energy source.For example, an external energy source can be connected directly acrosscapacitor 303 to power the low side integrated circuit 305 and the samesupply can be used to charge the capacitor 302 through a diode connectedbetween the energy source and the terminal 306, when switch 304 is on,to power the high side integrated circuit 304. This type of circuit,known as a bootstrap circuit, is well known to one skilled in the artand not shown in FIG. 3.

FIG. 4 shows another embodiment of a circuit benefiting from theteachings of the present invention. Voltage sense circuits 311 and 334are replaced by dv/dt sense circuits 411 and 434. For clarity thedescription below focuses on the high side dv/dt sense circuit 411. Theoperation of low side dv/dt sense circuit 434 can be assumed to beidentical.

As shown in the illustrated embodiment, instead of sensing the voltageacross switch 415, the dv/dt sense circuit 411 senses the slope of thevoltage across switch 415 over time while it is off. By sensing a slopeof the voltage across switch 415 in the off state, control circuitry 421can turn the switch 415 from an off state to an on state when the slopeof the voltage across the switch 415 changes. In one embodiment, thedv/dt sense circuits 411 is designed to sense when the slope of thevoltage across switch 415 changes polarity while it is in the off state.

A change in polarity of the slope of the voltage across switch 415 whileit is off identifies the point where the voltage across switch 415 hasreached a minimum and is about to start rising. It is beneficial tosense this point since a switching loss resulting from turning a switchfrom an off state to an on state is minimized when the voltage acrossthe switch is at a minimum value. All other details such as introducinga delay time between sensing the change in slope of the voltage acrossthe switch and turning a switch on and also sensing of a voltagethreshold during the switch on time period to detect fault conditionsapply in the same way as described above with reference to FIG. 3 andare therefore not repeated here.

In either the circuit of FIG. 3 or FIG. 4, both low side and high sideswitches are controlled as a function of the voltage across theswitches. In the case of FIG. 3, there is a first threshold voltagedetected while the switch is in the off state and used to control thetime at which the switch is turned on, and a second voltage thresholdused to detect a fault condition during the on time period of theswitch. In the embodiment of FIG. 4, the slope of the voltage across theswitch is detected while the switch is in the off state and used tocontrol the time at which the switch is turned on, and a voltagethreshold used to detect a fault condition during the on time period ofthe switch.

The circuits of FIGS. 3 and 4 do not show any providing of a feedbackcontrol loop from the output terminals 330 and 331 in FIGS. 3 and 430and 431 in FIG. 4 to the high side or low side control circuits. Sincethe switches of half bridge circuits described in FIGS. 3 and 4 are onlycontrolled as a function of the voltage across the switches, the powerdelivered to the load can only be regulated by adjusting the inputvoltages 327 and 427. However, FIG. 5 shows another embodiment of acircuit benefiting from the teachings of the present invention where afeedback control loop is included.

As shown in the embodiment of FIG. 5, a feedback control circuit 538provides a feedback signal 539, which provides information to the lowside integrated circuit 505 regarding an output of the power supply,typically the output voltage 529 or a current flowing in outputterminals 530 or 531. In the embodiment of FIG. 5, the feedback signal539 is coupled to low side integrated circuit 505 through a feedbackterminal 540, which then couples to a variable delay circuit 541internal to the control circuit 535. In other embodiments, the feedbackcontrol signal could also be coupled to the high side integratedcircuit, which would then include the variable delay circuit instead ofthe low side integrated circuit.

Taking the same initial condition as used to describe the circuitoperation in FIG. 3, when low side switch 524 turns off, the voltage atnode 514 rises with respect to terminal 528. In the embodiment shown inFIG. 5, high side control circuit 521 senses the change in slope of thevoltage across switch 515 while it is in the off state, though a voltagesense circuit 311 as shown in FIG. 3 could also be used.

After a delay time or deadtime, which is introduced by sense circuit511, the high side switch 515 is turned on for an on time period that issubstantially fixed by one shot circuit 509. When the high side switch515 turns off, the low side dv/dt sense circuit 534 senses the slope ofthe voltage across low side switch 524. When the sense circuit 534provides the signal, which would be coupled directly to the one shotcircuits 333 and 433 in FIGS. 3 and 4, respectively, it is insteadcoupled to a variable delay circuit 541. The variable delay circuit 541introduces a further delay the length of which is responsive to thefeedback signal 539, before one shot circuit 533 is allowed to turn onswitch 524.

In one embodiment, the additional delay time introduced by variabledelay circuit 541 is used to regulate the power delivered to the load.For example the delay time introduced by variable delay circuit 541 isextended if the feedback circuit 538 provides feedback that less poweris required by the load. This additional variable delay time requiresthat output filter inductor 537 is required to filter the outputvoltage. The circuits of FIGS. 3 and 4 did not show any output filterinductor of this type since the delay period between switching of highside and low side switches is a fixed delay or deadtime which isnormally minimized to reduce ripple in the output voltages 329 and 429.In other embodiments of the circuits of FIGS. 3 and 4, an output filterinductor may also be included depending on the output voltage ripplespecifications of a specific circuit.

FIG. 6 shows another embodiment of a circuit benefiting from theteachings of the present invention. For optimum operation of the powerconversion circuit 600, the magnitude of the voltage across thetransformer output winding 625 when high side switch 615 is in the onstate is substantially equal to the magnitude of the voltage acrosstransformer output winding 626 when low side switch 624 is in the onstate. This will minimize output voltage ripple between the high side ontime period and low side on time period.

In order for this condition to be satisfied, assuming that the voltagedropped across high side switch 615 when it is in the on state issubstantially equal to the voltage dropped across low side switch 624when it is in the on state, the voltage across high side capacitor 623is substantially equal to the voltage across low side capacitor 622. Inorder to meet this condition, the high side integrated circuit 604includes a load voltage sense circuit 637, which is coupled to receivetwo inputs. The first input is from node 639, which is coupled to thehigh side switch terminal 607, which in turn is coupled to the junctionof the low side, and high side switches 614. The second input 641 iscoupled to the junction 632 of the high side and low side capacitorsthrough resistor 640.

In the illustrated embodiment, the current flowing in resistor 640 issubstantially proportional to the voltage across the transformer winding618, which for the purposes of this description is considered the load.Therefore, in this configuration, the load voltage sense circuit 637 cansense the voltage across the input transformer winding 618, during thehigh side on time period when node 639 is coupled to the input terminal601 through high side switch 615. In one embodiment, voltage sensecircuit 641 can also sense the voltage across the load 618 during thelow side on time period when node 639 is coupled to the input terminal628 through the low side switch 624.

In order maintain the condition that the voltage across the load duringthe high side on time period is substantially equal to the voltageacross the load during the low side on time period, the high side ontime period in one embodiment is varied in response to an output signal638 from load voltage sense circuit 637, which is coupled to high sideone shot circuit 609. It will be known to one familiar in the art havingthe benefit of this disclosure that by varying the on time period ofhigh side switch 615 in this way, the energy delivered to and hencevoltage across capacitor 622 can be controlled to satisfy therequirement that the voltage across the transformer output winding 625when high side switch 615 is in the on state is substantially equal tothe voltage across transformer output winding 626 when low side switch624 is in the on state.

FIG. 7 shows a block diagram of one embodiment of a load voltage sensecircuit that will provide an output signal which is varied in responseto the relative magnitude of a voltage of a first polarity appliedacross a load during a first on time period and of the magnitude of anopposite polarity voltage applied across the load during a second ontime period. In one embodiment input terminals 701 and 703 are coupledacross the load. In one embodiment resistor 702 is equivalent toresistor 640 in FIG. 6. In another embodiment, resistor 702 could formpart of load voltage sense circuit 637 in FIG. 6 eliminating therequirement for external resistor 640.

In the illustrated embodiment, during the first on time period, forexample the on time period of high side switch 615, a first sense periodis initiated when switch 704 is switched on. The magnitude of current I1flowing in resistor 702, which is a sense signal, is substantiallyproportional to the magnitude of the voltage across the load, which isapplied as input voltage V_(LOAD) between terminals 701 and 703. Thecurrent I1 charges capacitor 708 relative to a voltage referenceterminal 705, which in one embodiment could be equivalent to terminal607. At the end of the first sense period, switch 704 is switched off.

During the second on time period, for example the on time period of lowside switch 624, a second sense period is initiated when switch 704 isagain switched on. The magnitude of current I2 which is a sense signal,is a function of the magnitude of the voltage across the load during theduring the second sense period. The current I2 discharges capacitor 708relative to reference voltage terminal 705.

If first and second sense periods are substantially equal in duration,the voltage Vout across output terminals 706 and 707 is substantiallyproportional to a difference in the magnitude of the voltage across theinput terminals 701 and 703 during the first on period and the voltageacross the input terminals 701 and 703 during the second on period. Inone embodiment, the voltage at terminal 706 relative to terminal 707 isequivalent to output signal 638 relative to terminal 607 in FIG. 6.

In the illustrated embodiment, the first and second sense periods areusually initiated sometime after the beginning of the first and secondon time periods respectively in order that ringing voltages and othertransient conditions, which occur at the time when a switch is firstswitched to an on state, have decayed to substantially zero, in orderthat accurate sensing can be achieved during the first and second senseperiods. Such transient conditions following the switching to an onstate will be familiar to one skilled in the art. In one embodiment, thesecond sense period is initiated sometime after the turn off of the highside switch 615 to allow sufficient time for the low side switch toturn-on and the transients to decay to substantially zero condition. Theoutput signal V_(DIFF) out of circuit 700 can be filtered as required toprovide a signal responsive to difference between the two polarities ofvoltage across the load.

In the foregoing detailed description, the present invention has beendescribed with reference to specific exemplary embodiments thereof. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of thepresent invention. The present specification and figures are accordinglyto be regarded as illustrative rather than restrictive.

1. A circuit for use in a half bridge converter, comprising: a high sideswitch coupled between a positive input terminal and a first terminal ofa primary transformer winding; a low side switch coupled between anegative input terminal and the first terminal of the primarytransformer winding; a first control circuit coupled to the high sideswitch, the positive input terminal and the first terminal of theprimary transformer winding, the first control circuit coupled to sensea slope of a voltage across the high side switch while the high sideswitch is off to control the high side switch in response to the sensedslope across the high side switch; and a second control circuit coupledto the low side switch, the first terminal of the primary transformerwinding and the low side input terminal, the second control circuitcoupled to sense a slope of a voltage across the low side switch whilethe low side switch is off to control the low side switch in response tothe sensed slope of the voltage across the low side switch.
 2. Thecircuit of claim 1 wherein the first control circuit is coupled to turnthe high side switch on for an on time period by generating a pulsehaving a pulse width of the on time period when the first controlcircuit senses the slope of the voltage across the high side switchchanges polarity while the high side switch is off.
 3. The circuit ofclaim 2 wherein the on time period of the high side switch issubstantially fixed.
 4. The circuit of claim 2 wherein the first controlcircuit is coupled to delay the high side switch from being turned onfor a delay time after the sensed slope of the voltage across the highside switch changes polarity while the high side switch is off.
 5. Thecircuit of claim 1 wherein the second control circuit is coupled to turnthe low side switch on for an on time period by generating a pulsehaving a pulse width of the on time period when the second controlcircuit senses the slope of the voltage across the low side switchchanges polarity while the low side switch is off.
 6. The circuit ofclaim 1 wherein the primary transformer winding is included in atransformer having secondary windings.
 7. The circuit of claim 6 whereinthe transformer includes a second terminal of the primary transformerwinding that is capacitively coupled to the positive input terminal andis capacitively coupled to the negative input terminal.
 8. The circuitof claim 7 wherein the second terminal of the primary transformerwinding is capacitively coupled to the positive input terminal through ahigh side capacitance and wherein the second terminal of the primarytransformer winding capacitively coupled to the negative input terminalthrough a low side capacitance.
 9. The circuit of claim 6 wherein thetransformer is included in a load.
 10. The circuit of claim 9 furthercomprising a feedback circuit coupled between the load and the secondcontrol circuit to provide a feedback control loop, wherein the secondcontrol circuit is coupled to vary a delay time to regulate powerdelivered to the load.
 11. The circuit of claim 1 wherein the high sideswitch is included in an integrated circuit.
 12. The circuit of claim 1wherein the first and second control circuits are included in anintegrated circuit.
 13. The circuit of claim 1 wherein the circuit isincluded in a power conversion circuit.
 14. The circuit of claim 1wherein the high side switch is a metal oxide semiconductor field effecttransistor (MOSFET).
 15. The circuit of claim 1 wherein the firstcontrol circuit is coupled to sense a slope of a voltage across thepositive input terminal and the first terminal of the primarytransformer winding while the high side switch is off to control thehigh side switch in response to the sensed slope across the positiveinput terminal and the first terminal of the primary transformerwinding.